1. Field of the Invention
The present invention relates to a surface light source device of side light type, and more particularly to a technique for improving uniformity of output light in the surface light source device. The present invention is applied, for example, to backlighting for a liquid crystal display.
2. Description of the Related Art
A surface light source device of side light type is conventionally applied to a liquid crystal display to illuminate a liquid crystal panel from its back surface. The arrangement is suitable for making an entire shape of the surface light source device thin.
A bar-shaped light source such as a cold cathode tube, etc. is generally adopted as a primary light source in the surface light source device of side light type and is arranged on the side of a guide plate (a guide body formed in a plate shape). Illuminating light emitted from the primary light source is introduced into the guide plate through a side end surface of the guide plate. The introduced illuminating light is propagated within the guide plate and light is emitted from a major surface of the guide plate toward a liquid crystal panel during the propagation.
It is known that the guide plate adopted in such a surface light source device of side light type is of a type having a substantially uniform plate thickness and a type having a tendency in which a plate thickness is reduced as distance between the guide plate and the primary light source is increased. In general, the latter efficiently emits illuminating light in comparison with the former.
FIG. 8 is an exploded perspective view showing a surface light source device of side light type using the guide plate of the latter type. FIG. 9 shows a cross section taken along line A—A of FIG. 8. With reference to FIGS. 8 and 9, the surface light source device 1 of side light type has a guide plate 2, a primary light source 3, a reflection sheet 4, a light diffusive sheet H and prism sheets 5, 6 serving as light control members. The primary light source 3 is made of a light source element 7 and a reflector 8. The elements except for the guide plate 2 and the primary light source element 7 are also called“additive members”. The additive members illustrated in FIGS. 8 and 9 are the refection sheet 4, the light diffusive sheet H, the prism sheets 5, 6 and the reflector 8.
The guide plate 2 is a transparent guide plate having a wedge-shaped cross section. For example, acrylic resin is adopted as a material of the guide plate 2. The guide plate 2 has two major surfaces 2B, 2C. One major surface 2C is employed to emit illuminating light and is called an emission. The other major surface 2B is called a back surface. For example, a light diffusive surface is formed on the back surface 2B by mat processing in a normal example.
The light source element 7 is, for example, a cold cathode tube (fluorescent lamp) and a reflector 8 having a nearly semicircular shape in cross section is arranged behind the light source element 7. Illuminating light is supplied to a side end surface 2A of the guide plate 2 through an opening of the reflector 8. A sheet-like regular reflection member made of a metallic foil, etc., or a sheet-like irregular reflection member made of a white PET film, etc. is adopted as the reflection sheet 4.
Illuminating light L from the primary light source 3 is introduced into the guide plate 2 through the incidence surface 2A and is then propagated toward a distal end of the guide plate 2 while the illuminating light is repeatedly reflected between the back surface 2B and the emission surface 2C. The reflection sheet 4 is arranged along the back surface 2B.
Incident angles of illuminating light to the emission surface 2C are reduced stepwise every reflection on the slope 2B. The reduction causes increase in light components having angle equal to or smaller than the critical angle with respect to the emission surface, and promotes light emission from the emission surface. Thus, deficiency of emission in an area far from the primary light source 3 is prevented.
Illuminating light emitted from the emission surface 2C has a property of scattering light since the illuminating light is further irregularly reflected on the back surface 2B having a light diffusive property, or the reflection sheet 4. However, a main propagating direction of illuminating light emitted from the guide plate 2 is inclined to a distal direction (a direction opposed to the primary light source 3) with respect to a frontal direction. Namely, the emitted light of the guide plate 2 has directivity. Such property of the guide plate is called directive emitting property.
The light diffusive sheet H is arranged to diffuse illuminating light emitted from the guide plate 2 and prevent a light diffusive surface of the back surface 2B from being visually recognized from above the emission surface 2C. Further, the light diffusive sheet H is arranged to prevent conspicuous glow and shade portions from appearing in the guide plate 2 lightened by illuminating light.
The prism sheets 5, 6 are arranged to correct emitting directivity of the guide plate 2. For example, the prism sheets 5, 6 are formed by a transmissive sheet material such as polycarbonate. In many cases, the prism sheets 5, 6 are arranged so that a prism surface of the prism sheet 5 is opposite with the guide plate 2 and a prism surface of the prism sheet 6 also is opposite with the guide plate 2.
Each of the prism surfaces has many projections having a triangular shape in cross section and extending in an approximately parallel direction. The inside prism sheet 5 is orientated so that its projections run in parallel with the incidence surface 2A. The outside prism sheet 6 is orientated so that its projections run in a direction approximately perpendicular to the incidence surface 2A.
Slopes of the projections correct the main emitting direction of the emitted light to a frontal direction of the emission surface 2C. A so-called double-sided prism sheet in which prism surfaces are formed on its both sides may be used.
In general, a surface light source devices of side light type adopting such a wedge-shaped guide plate and the prism sheets emit light in the frontal direction more efficiently than a surface light source device of side light type adopting a guide plate having a substantially uniform thickness.
However, an undesirable bright line appears on the guide plate 2 in the conventional device explained above so that uniformity of output light is reduced. As illustrated by reference sign K in FIG. 8, one to several bright lines are generated as a local thin high luminance band on the emission surface 2C near the incidence surface 2A. If a very high light diffusive property is given to the light diffusive sheet H, such bright lines are inconspicuous, but an entire reduction in luminance is caused.